package/mpir: force arm mode instead of Thumb mode

Fix the following build failure:
/tmp/ccuwJnnt.s:124: Error: selected processor does not support `umull ip,r6,r2,r1' in Thumb mode
/tmp/ccuwJnnt.s:159: Error: selected processor does not support `umull ip,r0,r7,r1' in Thumb mode

Fixes:
http://autobuild.buildroot.net/results/610/61094bc7b7eb1d1e49bf8fcd1864b5cd3d7c4499/

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit ff037fe5430738e85d9c2270883e71cd77e99ff3)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mpir/mpir.mk

@@ -36,4 +36,8 @@ ifeq ($(BR2_powerpc64le),y)
 MPIR_CONF_ENV += MPN_PATH="generic"
 endif
 
+ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
+MPIR_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -marm"
+endif
+
 $(eval $(autotools-package))